In alternator based charging systems, such as those used in automotive applications and the like, it is often necessary to detect a state of the alternator rotation for control purposes etc. Conventional detection circuits typically use the voltage signals from two phases of the alternator in order to reject the DC common mode voltage that can occur on the phase signals output by the alternator. For example, and as illustrated in FIG. 1, a typical three phase alternator regulator 100 generates three phase signals 112, 114, 116, the three phase signals 112, 114, 116 comprising cyclic voltages that are shifted in phase by 120 degrees relative to one another. Two of the three phase signals 112, 114 are provided to a detection component 120. FIG. 2 illustrates the two phase signals 112, 114 received by the detection component 120, which in FIG. 2 comprise a 3V common mode voltage. The detection component 120 derives a difference voltage, illustrated at 210 in FIG. 2, between the two phase signals 112, 114. The difference voltage 210 is immune to the DC common mode voltage, whilst having the same frequency as the individual phase signals 112, 114, 116 and thereby enabling the state of the alternator rotation to be detected without interference from any common mode voltage present within the two phase signals 112, 114.
Customer requirements and demands have given rise to a continued drive towards alternator rotation detection solutions that are lower cost, are less prone to defects and have better immunity to noise. Any such solution must also compensate for DC common mode voltages within the phase signals.